media: radio-si470x: make si470x_viddev_template const

Make this const as it is only used in a copy operation in the files
referencing it. Add const to declaration in the header too.

Structure found using Coccienlle and changes done by hand.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>